Zinc purification



United States Patent Office 3,169,855 Patented Feb. 16, 1965 3,169,855 ZINC PURIFICATION Erich Pelzel, Stolherg, Rhineland, Germany, assignor to StolbergerZink Aktiengesellschaft, Aachen, Germany No Drawing. Filed Sept. 27, 1962, Ser. No. 227,651 Claims priority, application Germany, Aug. 22, 1961,

9 Claims. ((11. 75-88) The present application is a continuation-in-part of the US. patent application Serial No. 217,517, entitled Zinc Purification, filed on August 15, 1962, now abandoned, and of the US. patent application Serial No. 218,451, entitled Z-inc Purification, filed on August 21, 1962, now abandoned.

The present invention relates to a method of purifying zinc and, more particularly, to a method of reducing the lead content of Zinc and of certain zinc alloys.

Thus, the present invention is also concerned with a method of producing zinc of relatively high purity from smelter zinc or remelt zinc.

According to the German standard DIN 1706, the various grades of smelter zinc and remelt zinc will contain between 96% and 99.5% of zinc and, as principal impurity, lead in amounts of between 0.5% and 2.4% or even higher.

It is known to reduce the lead content of such zinc to between about 1 and 1.3% by the so-called refining by liquation (Zinktaschenbuch, Metallverlag, 1959, page 47).

However, zinc and zinc alloys of greater purity, i.e., lower lead content are required for many purposes. Thus, for instance, so-called mixed zincs containing between about 0.2% and 0.7% lead, are produced by mixing smelter zinc and highly refined zinc such as zinc of 99.99% purity.

It is obviously an expensive and uneconomical method to use such highly refined zinc for producing a mixed zinc of lesser purity. g

It is therefore an object of the present invention to provide a methodfor reducing the lead content of lead-containing zinc and of certain lead-containing zinc alloys in a simple and economical manner.

Other objects and advantages of the present invention will become apparent from a further reading of the description and of the appended claims.

With the above and other objects in view, the present invention comprises in a method of reducing the lead content of lead-containing zinc, the steps of forming a molten mixture having a temperature of at least about 470 C. of the lead-containing zinc and of a substance selected from the group consisting of sodium andpotass'ium, cooling the molten mixture to a temperature above and in the vicinity of the melting point of zinc, thereby forming on the surface of the molten mixture a dross comprising a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below the dross, and separating the thus formed molten zinc of reduced lead content from the dross.

According to a preferred embodiment, the present invention contemplates in a method of reducing the lead content of'lead-containing zinc alloys including in the indicated proportions at least one alloying element selected from the group consisting of up to 10% aluminum, up to copper, up to 1% calcium and up to 1% magnesium, the steps of forming a molten mixture having a temperature of between abount 475 and 500 C. of the .leadcontaining Zinc alloy andof between about 0.3% and 2.6% based on the weight of said molten mixture of sodium, cooling said molten mixture to a temperature of about 430 C., thereby forming on the surface thereof a dross comprising a relatively high proportion of lead while simultaneously reducing the lead ,content of the molten zinc alloy located below the dross, and separating the thus formed molten zinc alloy of reduced lead content from the dross.

In view of the fact that, for instance, sodium forms with zinc a compound such as NaZn which has a considerably higher melting point than zinc and that it is possible to form other intermetallic compounds between zinc and alkali metals or alkaline earth metals which intermetallic compounds have higher melting points than zinc, it seemed at first as if it might be possible to influence a liquation process in such a way that zinc would be caused to liquate or segregate in the form of such intermetallic compounds of higher melting points and thus to be separated from a molten lead-containing residue of lower melting point. In other words, it was first expected that by adding sodium to a lead-containing smelter zinc, a high melting-point sodium-zinc compound would segregate upwardly from the melt and thus would be separated from the residual melt having a high lead content and being of higher specific gravity.

However, surprisingly, it was found according to the present invention that when, for instance, zinc meltswith a lead content of between 1% and 1.2% were alloyed at about 500 C. with sodium and cooled, after the sodium had been admixed, to about 430 0, Me. somewhat above the melting point of zinc, dross of high lead content formed on the surface of the melt and the residual 'melt below the dross contained a much smaller proportion of lead.

Thus, by proceeding in this manner and casting plates of the residual melt, which plates where then analyzed, it was found that with increasing within certain limits the amount of sodium an increasing reduction in the lead content of the molten zinc was achieved. Somewhat. similar results were also achieved when the sodium was re placed by potassium.

The following Table I shows the decrease in the lead content of the residual melt after removal of the dross, dependent on the proportion of sodium added to the original melt;

Table I Lead content of residual I melt after removal of dross Sodium in percent by weight of total melt 000. 00 HNJJBU ECOHOOOOH Gamma. O0 0 0 O O O 0. 00r .OHMtPCDOON) .DJU'IOCDOOQ By further increasing the proportion of sodiumadded to the melt, it was possible to reduce the lead content of the residual melt to below 0.01%. i

It is surprising that the zinc is not preferentially combined as an intermetallic dross-forming phase with the added sodium. By way of explanation,'but without limiting the invention to such explanation, it may be assumed that a ternary Na-PbZn phase of lower specific weight is formed and liquates upwardly, thus making it possible, in accordance with'the present invention, 'to' remove lea from zinc in an economical and simple manner.

Thus, according to the present invention, zinc of relatively high purity and witha lead content below 1% and even very much lower can be produced from, for instance, smelter or remelt zinc having a lead content higher than 1% This is accomplished by alloying or admixing to the V '5 V zinc melt sodium or potassium, preferably at a temperature of tli'e melt of between about 470 and 520 C. and most preferably between 475 and 500 (2., whereby the amount of sodium or potassium added will depend on the degree of leadremoval which is desired and may range upto about of the total weight ofthe melt. Larger proportions of the alkali metal usually are not required. i

The zinc melt containing the sodium or potassium is then cooled down to about 430 C. and at about that temperature a dross will form on the surface of the melt containing a high proportion of lead-while the lead content of the residual zinc melt below the drosswill be correspondingly reduced. The dross can then be removed "and the zinc of reduced lead content cast into plates or otherwise further processed. I

In most cases it will not be necessary to use more than 2.6% of sodium. 7

As stated above, the preferred temperature of the melt will be between 470 C. and 520 C. 'At temperatures higher than 557 C. and with a sodium content of the melt of up to 2.64%, a homogeneous melt is formed and no separation takes place. At temperatures higher than 557" C. and with sodium contents exceeding 2.64%, a

homogeneous zinc-sodium-lead melt is formed with a layer of undissolved liquid sodium on top thereof. Only when the temperature is below 557 C. and particularly when the temperature is maintained in the preferred range of bctween' 520" C. and 470 C., a liquidlead-sodium-zinc phase is formed and will rise to the top of the melt. The major portion of the lead which is to be removed will separate upwardly, after sodium addition within the pre ferred temperature range, in the form of a liquid leadsodium phase containing only a smallproportion of zinc (less than 10%). a Upon further cooling toward the melting point of zinc, the solid compound NaZn will be precipitated and rise to'the top of the melt, thereby reducing the residual sodium content of the melt. The foregoing is further described in Example 11.

The examples forming part of this specification are given as illustrative only Without limiting the invention to the specific details of the examples.

EXAMPLE I According to the presentexample, zinc containing 05% lead was to be prepared from a smelter zinc having a lead content of 1.11%.

3,300 kg. of zinc were treated at a temperature of 47 5 7 C. by the addition thereto of kg. of sodium, i.e. an amount of sodium equal to 0.6% of the zinc. After mixing the sodium into the melt, the latter was cooled to 430 C. 89 kg. of dross separated on the surface of the melt and were skimmed off. The total amount of dross thus was equal to 2.7% of the batch. In. this and several Calcium alone or mixtures of calcium and magnesium support the action 'ofsod-ium or potassium so that by adding calcium or mixtures of calcium and magnesium an amount of sodium or potassium will be required to achieve a predetermined leadelimination effect which is less than the amountof sodium or potassium whiehwould be required in the absence of calcium or calcium and magnesium. Copper raises the specific weight of the melt so that the lighter, lead-containing phase can be more easily separated. Thus, the specific manner in which the process is to be performed may be controlled by operating with or without any one of the metals aluminum, calcium,

magnesium and copper.

The metals aluminum, magnesium and copper are. frequently present in suflicient quantities in the original zinclead melt, While calcium or calcium and magnesium (as well as aluminum, magnesium and copper when not contained in the original zinc melt) may be stirred into the melt together with the sodium.

' Residual sodium retained in the purified zinc melt in a quantity, of less than 0.1% may be easily removed by various processes known per se, such as by oxidation with air, .with sodium hydroxide, zinc chloride or ammonium chloride.

It has been found that traces of sodium remaining in a zinc refined according to the present invention will improve the mechanical properties thereof during casting and kneading (cold deformation) operations. Zinc, of

low lead content, produced according to the present invention, has been found to be particularly suitable for galvanizing iron and steel and also for the manufacture of zinc alloys containing aluminum," copper and titanium. EXAMPLE II The lead content of the original zinc melt was 1%, and

0.6% of sodium were introduced at a temperature of Thereafter, the melt was cooled to 47 5 C. and the supernatant lead-rich liquid phase formed thereby was removed. The melt now contained 0.5% lead and 0.18% sodium. Upon further cooling towards the melting point of zinc, practically down to 430 C.,' zinc-sodium crystals rose to the surface of the melt and were-removed, whereupon the residual melt contained 0.5% lead and less than 0.004% sodium;

EXAMPLE III The lead content'of zincoriginally containing 1.0% 0

lead and 0.3% aluminum and intended for use in galvanizing ironwas to be reduced to 0.3%.

melt while the same was maintained at a temperature of- .500" C. Thereafter, the melt was cooled to 430 C. and the supernatant liquid lead-sodium and solid zinc-sodium other experiments, the dross contained:

Table II Percent- Lead 20-25 Zinc 40-45 Sodium 18-20 Bound oxygen Balance The lead content of thethus refined zinc was 0.50% and the residual sodium content of the zinc was below 0.1%. The residual sodium'could be removed by treatmerit-with ammonium chloride.

By computing the costs of 6 kg. of sodium per metric ton of zinc plus loss of zinc in the dross, it is found that the method according to the present example is considerably more economical than to produce such mixed zinc, i.e. zinc containing about 0.50% lead from a mixture of smelter zinc andrefined zinc. V

Substantially similar results were obtained when the sodium was wholly or in part replaced by potassium. Moreover, the removal of lead from smelter zinc or the following composition:

.For this purpose, 10 kg. of sodium were added to the phases formed'upon cooling were removed. V

The residual zinc melt was cast into-ingots and was. found to contain 0.2% aluminum, 0.27% lead and 0.004%

sodium.

The total dross which had been removed from the surface of the zinc melt amounted to 29 kg. andihad the Table III Percent Lead 23.8 Zinc 26.9 Sodium 30.6 Aluminum 1.01

Oxygen Balance.

1,000 kg. of zinc containing 1.2% lead were to be transformed into an intermediate quality of so-called mixed zinc which was to contain about 0.6% lead.

For this purpose, *6 kg. of sodium were stirred into the zinc melt while the same was maintained at 520 C. The melt was then cooled down to 470 C. and, after one hour, 20 kg. of a lead-rich dross were removed. The melt was then further cooled to a temperature of 430 C. and one hour later 12 kg. of a zinc-rich dross were removed. Thereafter 1 kg. of zinc-muriate ammonia was added to the melt and removed therefrom one hour later. The residual melt was then cast into ingots and was found to contain 0.55% lead and 0.001% sodium.

EXAMPLE V Table IV Percent Lead 20.8 Sodium 26.5

Zinc 35.2

Oxygen Balance EXAMPLE VI The lead content of a zinc melt containing 0.35% lead and 0.2% aluminum had to be reduced to less than 0.1%

1 kg. calcium and 3 kg. sodium were added and thereafter the melt was cooled to 470 C. At this temperature a lead rich dross was removed and the melt then further cooled to 430 C. Thereafter a zinc-rich dross was removed and the residual melt treated with sodium hydroxide for the removal of sodium therefrom.

The thus obtained Zinc melt contained 0.08% lead, 0.1% aluminum and only traces of sodium and calcium.

EXAMPLE VII 1000 kg. of zinc (prime western quality) having a lead content of 1.5% had to be purified in a single step operation to a lead content of less than 0.1%.

For this purpose, 30 kg. of sodium were stirred into the melt while the same was maintained at a temperature of 520 C. and this was followed by cooling the melt to 430 C. After two hours, the dross was removed and the residual melt washed with Zn--NI-I Cl. The thus purified zinc contained 0.07% lead and 0.001% sodium. The total dross which had been removed from the surface of the zinc melt amounted to 68 kg. and had the following composition:

EXAMPLE VIII A zinc alloy which was to be rolled and which contained 1.0% aluminum and 1.0% copper showed intercrystalline corrosion caused by the presence of 0.3% lead. a

1000 kg. of the alloy were treated at a temperature of 500 C. with 4 kg. sodium plus 1 kg. potassium.

The melt was then cooled to 425 C., the dross removed and the residual melt cast into ingots.

The thus purified alloy did not show any intercrystalline corrosion. A total of 980 kg. of purified alloy was recovered, containing 0.8% aluminum, 1.1% copper, 0.01% sodium, traces of. potassium and 0.03% lead.

The dross amounted to 18 kg. and had the following composition:

Table VI Percent Lead 16.7 Aluminum 11.0 Sodium 27.8 Zinc 27.7 Oxygen Balance Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristicsof the generic or specific aspects of this invention, and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

The embodiments of the invention in which an excluisive property or privilege is claimed are defined as folows:

1. In a method of reducing the lead content of zinc containing a minor proportion of lead, the steps of forming a molten mixture having a temperature of at least about 470 C. but less than 557 C. of said lead-conraining zinc and of an effective amount of a substance selected from the. group consisting of sodium and potassium; cooling said molten mixture to a temperature in the vicinity of but not below the melting point of zinc, thereby forming on the surface of said molten mixture a dross containing zinc, said substance and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below said dross; and separating the thus formed molten zinc of reduced lead content from said dross.

2. In a method of reducing the lead content of zinc containing up to about 4% lead, the steps of forming a molten mixture having a temperature of between about 470 and 520 C. of said lead-containing zinc and of an effective amount of a substance selected from the group consisting of sodium and potassium; cooling said molten mixture to a temperature of about 430 C., thereby forming on the surface of said molten mixture a dross containing zinc, said substance and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below said dross; and separating the thus formed molten zinc of reduced lead content from said dross.

3. In a method of reducing the lead content of zinc containing lead as principal impurity, the steps of forming a molten mixture having a temperature of between 475 and 500 C. of said lead-containing zinc and of an effective amount of sodium; cooling said molten mixture to a temperature of about 430 C., thereby forming on the surface of said molten mixture a dross containing zinc, sodium and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below said dross; and sepa rating the thus formed molten zinc of reduced lead content from said dross.

4. In a method of reducing the lead content of zinc containing up to about 2.4% lead, the steps of forming a molten mixture having a temperature of between about 470 and 520 C. of said lead-containing zinc and of an elfective amount of sodium being equal to up to 5% of the weight of said molten mixture; cooling said molten mixture to a temperature of about 430 C., thereby forming on the surface of said molten mixture a dross containing zinc, sodium and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below said dross; and separating the thus formed molten zinc of reduced lead content from said dross.

5. In a method of reducing the lead content of zinc containing a minor proportion of lead, the steps of forming a molten mixture having a temperature of between about 470 and 520 C. of said lead-containing zinc and of sodium in an amount equal to between about 0.3%

and of the weight of said molten mixture; cooling said molten mixture to a temperature of about 430 (3., thereby forming on the surface of said molten mixture at dross containing zinc, sodium and a relatively high proportion of lead while simultaneously reducing the lead a dross containing zinc, sodium and a relatively high proportion of lead while simultaneously reducing the "leadcontent of the molten zinc located below said dross;

and separating the thus formed molten zinc of reduced leadcontent from said dross. 7. In a method'of reducing the lead content of zinc containing a minor proportion of lead, the steps of forming a molten mixture having a temperature of between about 453 and 500C. of said lead-containing zinc and of an effective amount of sodium beingtequal to up to 2.6% of the weight of said molten mixture; cooling said molten mixture to a temperature of about 430 C.,' thereby forming on the surface of said molten mixture at dross containing zinc, sodium and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc located below said dross;

and separating the thus formed molten zinc of reduced lead content from said dross. Y a

8, In a method of reducing the lead content of zinc alloys containing as principal impurity a minor propor- 7-15 content of the molten zinc located below said dross;

consisting of up to 10% aluminum, up'to 5% copper,

up to 1% calcium and up to 1% magnesium, the steps of forming a molten mixture having a temperature of between about 470 and 520? vC. of said lead-containing zinc alloy and of an effective amount of sodium; cooling said moltentrnixture to a temperature of about 430 0., thereby forming on the surface thereof atdross containing zinc sodium and a relatively high proportion of 'lead while simultaneouslyrreducing the lead content of the molten zinc alloy located below said dross; and

separating the thus formed molten zinc alloy of reduced lead content from said dross. i v V 9. In a method of reducing the lead content of zinc alloys containing'lead as principal impurity and including in the indicated proportions at least one alloying element selected from the group consisting of up to 10% aluminunnup to 5% copper, up to 1% calcium and up to 1% magnesium, the steps of forming a molten mixture having a temperature of between about 453 and 500 C. of said lead-containing zinc alloy and of between about 0.3% and 2.6% based on the weight of said molten mixture of sodium; cooling said molten mixture to pa temperature of about 430 (3., thereby forming on the surface thereof a dross containing zinc, so

diurn and a relatively high proportion of lead while simultaneously reducing the lead content of the molten zinc alloy located below said dross; and separating the thus formed molten zinc alloy of reduced} lead content from said dross.

References Cited by the Examiner UNITED STATES PATENTS tion of lead and including in the indicated proportions V at least one alloying element selected from the group FOREIGN, PATENT 7/60 Canada.

' OTHER REFERENCES Hansen: Constitution of Binary Alloys, 2nd ed.,'

McGraw-Hill Book Co., Inc, New York, pages 99, 884 and'1l19 relied on. a 1 V tMathewson: Zinc, The Metal, Its Alloys and Compounds, Reinhold Publishing Corp, 1959, pages 335-338 V and pages627-632 relied on. x f

, EENJAMrN HENKIN, Primary Exam ner.

DAVlD L. RECK, Examiner. 

1. IN A METHOD OF REDUCING THE LEAD CONTENT OF ZINC CONTAINING A MINOR PROPORTION OF LEAD, THE STEPS OF FORMING A MOLTEN MIXTURE HAVING A TEMPERATURE OF AT LEAST ABOUT 470*C. BUT LESS THAN 557*C. OF SAID LEAD-CONTAINING ZINC AND OF AN EFFECTIVE AMOUNT OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM; COOLING SAID MOLTEN MIXTURE TO A TEMPERATURE IN THE VICINITY OF BUT NOT BELOW THE MELTING POINT OF ZINC, THEREBY FORMING ON THE SURFACE OF SAID MOLTEN MIXTURE A DROSS CONTAINING ZINC, SAID SUBSTANCE AND A RELATIVELY HIGH PROPORTION OF LEAD WHILE SIMULTANEOUSLY REDUCING THE LEAD CONTENT OF THE MOLTEN ZINC LOCATED BELOW SAID DROSS; AND SEPARATING THE THUS FORMED MOLTEN ZINC OF REDUCED LEAD CONTENT FROM SAID DROSS. 